To mark the 25th anniversary of the mayoral election between Sidney Barthelemy and Bill Jefferson, the Louisiana Endowment for the Humanities recently screened documentarian Paul Stekler's account of the showdown, "Among Brothers."

As the title suggests, the film focuses on the historic nature of that 1986 contest, the first mayoral runoff in New Orleans history between two African-American candidates. One underlying lesson of the movie is that white voters were able to tip the election to one of the two black candidates -- in this case, Barthelemy.

Although the movie suggests this was a novel concept at the time, it reflected a classic political dynamic.

Until recently, Louisiana statewide elections also had a recognizable swing electorate.

With Republicans rising in north Louisiana and the suburbs and Democrats still strong in major cities, elections were for years decided in Acadiana, an area dominated by socially conservative but more economically populist voters.

It's no coincidence that so many successful Democratic pols hailed from there, people like former Gov. Kathleen Blanco and retired U.S. Sen. John Breaux. And it's no accident that the party kept going back to the same well in search of more Democrats who might attract right-leaning voters, even after the dynamic had played itself out.

Now that the GOP has become so dominant that the Democratic Party can't even field candidates for major offices, there's a new swing electorate in Louisiana: Democratic voters. As we saw in last month's statewide elections, the landscape gives this nominally powerless minority some real power.

And that could spell trouble for those GOP politicians who veer too far to the right, even in a state that votes overwhelmingly Republican. In the case of arch-conservative U.S. Sen. David Vitter, it may already have.

By any standard, the senator has made an extraordinary comeback since he hit bottom in 2007, the year his prostitution scandal broke. Not only did he survive, he triumphed when he ran for reelection last year and trounced Democrat Charlie Melancon -- one of those mythical Cajun "John Breaux Democrats"-- by linking him to an unpopular Democratic president.

Vitter's next move was to try to spread his brand of ideological purity to Baton Rouge by aggressively backing a slate of candidates for state office, including in-party challengers to Republican incumbents for lieutenant governor and secretary of state.

That his candidates fell short in both races suggests the new dynamic has already set in.

Actually, the new reality revealed itself even before the votes were counted.

Even as Lt. Gov. Jay Dardenne and his Vitter-backed opponent Billy Nungesser burnished their conservative credentials, each also went after Democratic votes.

Nungesser, the Plaquemines Parish president, made a strong play for traditionally Democratic African-American voters. He courted endorsements, landed spots on sample ballots of New Orleans political organizations LIFE and BOLD and even ran targeted radio ads touting his respect for President Barack Obama, the same man Vitter attacks at nearly every turn.

But Dardenne had an easier task. While he too sought and won African-American endorsements, including one from COUP, he mostly fell back on his moderate record as a state senator, secretary of state and lieutenant governor. In fact, he didn't even have to advertise his relative centrism, because his adversaries did it for him.

Vitter's prominence in Nungesser's camp in the first place sent a message. So did Nungesser's attacks against Dardenne from the right, which were designed to let conservative voters know he's one of them, but also sent a message to moderates and liberals that he's not.

If Vitter is indeed thinking of coming back to Louisiana to run for governor in a few years -- against Dardenne, potentially-- he could face this suddenly tricky terrain. Just call it the hidden cost of the GOP's unprecedented success.